If you mix Oxalic Acid (which you can buy legally from E-Bay for about £10 per Kilo) with water and wallpaper paste you get a sticky gel which can be applied with a paint brush to the desired area for cleaning. Apparently, 1kg of Oxalic Acid will make enough gel for 2-3 6m boats

It takes about 5 minutes for the gel to work and after that time you can simply wash the gel off with plenty of cold water.

Once it has been washed off, you should allow the area to dry and then protect it with a marine polish.

This works particularly well for boats that have been left on a wet berth without anti-foul, or for parts of an anti-fouled boat that are just above the waterline. It also works well for removing rust marks around engine and seat fittings It certainly saves a lot of scrubbing and negates the need to buy expensive "marine stain removers".